1. Field of the Invention
The invention relates to a variable valve unit for an internal combustion engine which variable valve makes the driving phase of an intake or exhaust valve to be variable.
2. Description of the Related Art
For the purposes of, for example, improving engine exhaust gas countermeasures and reducing fuel consumption, there are cases a variable valve unit is mounted in an engine as an internal combustion engine mounted in a vehicle. The variable valve unit varies the phases of intake and exhaust valves, namely, the valve-opening/closing timing of the intake and exhaust valves corresponding to the operation mode of a vehicle.
As a structure of a variable valve unit of the above-described type, a reciprocating-cam type structure is known. The reciprocating-cam type structure is a structure in which the phase of a cam formed to a camshaft is displaced by a reciprocating cam in which a base circle interval and a lift interval are connected.
A large number of reciprocating-cam type structures of the above-described type employ a rocker arm mechanism that makes the ratio between the base circle interval and the lift interval transferred to the reciprocating cam to be variable. The rocker arm mechanism causes the ratio to be variable corresponding to the operation mode of a vehicle. A variable valve unit of the above-described type is disclosed in Japanese Patent No. 3245492, for example.
In addition, for engines, pumping loss is required to improve fuel to attain fuel consumption reduction.
When the pumping loss is taken into account, when making the intake valve to be variable, the phase, namely, valve-opening/closing timing of the intake valve is preferably made variable while the closing time of the intake valve is substantially maintained. Thereby, intake air is drawn into a cylinder.
However, according to the variable valve unit disclosed in Japanese Patent No. 3245492, the cam phase of the camshaft is simply transferred to the reciprocating cam. Accordingly, a portion corresponding to a maximum lift amount of the cam phase made variable is substantially coordinated with the state of cam phase before being made variable. Then, the valve-opening time and the valve-closing time of the cam phase made variable vary with respect to the state before being made variable.
An engine in which such a reciprocating-cam type variable valve unit as described above, therefore, concurrently uses a variable valve unit of a type different from the reciprocating-cam type variable valve unit, in combination therewith.
More specifically, the engine concurrently uses a variable valve unit that uses hydraulic forces to displace the cam itself along, for example, an advancing angular direction or a retarding angular direction. The variable valve unit makes the intake valve phase to be variable so that the valve-opening time is substantially maintained constant, thereby to reduce the pumping loss.
However, the plurality of variable valve units are used as described above, the both variable valve units should be properly controlled at the same time. In addition, also a phase variability amount should be large, so that an undesirable case can perhaps occur in which the response characteristics, variability amount, and the like become insufficient, thereby causing fuel economy to remain unimproved.